Superconductivity in the dilute single band limit in reduced Strontium Titanate

Abstract

We report on superconductivity in single crystals of SrTiO${}_{3-\hat{I}´}$ with carrier densities $\text{n}<1.4\times 10^{18}c{m}^{-3}$, where only a single band is occupied. For all samples in this regime, the resistive transition occurs at $T_c\approx 65\pm 25mK$. We observe a zero resistance state for $\text{n}$ as low as $1.03\times 10^{17}c{m}^{-3}$, and a partial resistive transition for $\text{n}=3.85\times 10^{16}c{m}^{-3}$. We observe low critical current densities, relatively high and isotropic upper critical fields, and an absence of diamagnetic screening in these samples. Our findings suggest an inhomogeneous superconducting state, embedded within a homogeneous high-mobility 3-dimensional electron gas. $T_c$ does not vary appreciably when $\text{n}$ changes by more than an order of magnitude, inconsistent with conventional superconductivity.

5 pages, 4 figures